A disc drive storage system 10, illustrated in FIG. 1, comprises a disc 12 further comprising a magnetic material for storing information in the form of binary bits for later retrieval and processing by a computer or processing device. Information is written to the disc 12 by magnetizing magnetic domains within the magnetic material to represent a binary zero or a binary one. The domains retain the magnetization for later retrieval during a disc read operation.
A spindle motor 13 rotates the disc 12 (typically at speeds up to 10,000 revolutions per minute) allowing a read/write head 14 to write or read data as the read/write head 14 flies over an upper surface of the disc 12. The read/write head 14 is affixed to an actuator and suspension arm 16 controlled by a voice coil motor 18 for moving the suspension arm 16 across the upper surface of the disc 12 along an arc extending between a disc circumference 24 and a hub 26. The physical features of the suspension arm 16 cause the read/write head 14 to ‘fly’ very close to the disc upper surface, as head-to-disc contact is undesired.
Certain head embodiments conventionally comprise two separate transducing elements (not shown in FIG. 1), an inductive writer and a magnetoresistive (MR) reader. Earlier-generation heads utilize a single inductive transducer for both reading and writing. The present application, in the interest of clarity, assumes use of a dual-element head, however the invention is not limited in application to a dual-element head embodiment.
The disc 12 comprises a plurality of concentric tracks 30 (typically 20,000 per radial inch) for interlaced storage of binary-encoded user data in fields 32 and head location data in servo bursts 34. The servo bursts 34 (typically 200 per disc track), which are radially contiguous across the disc 12 and equally-spaced circumferentially along each track, provide feedback information to the read/write head 14 for accurately controlling head position along the track (referred to as track following) and for moving the read/write head 14 rapidly and accurately between tracks (referred to as track accessing).
To write data to the disc 12, the voice coil motor 18 moves the suspension arm 16 to a desired radial position above the surface of the disc 12. The disc 12 is rotated to move a circumferential region to be written under the read/write head 14. Write current is supplied to a coil (magnetically coupled to a magnetically permeable core) of the head's inductive writer to induce a magnetic field in the core. The magnetic field extends from the core across an air gap between the read/write head 14 and the disc 12 to magnetize a small region of magnetic domains to store the data bit. The direction of the magnetic field produced by the head, and thus the direction of the magnetic domains, is dependent on the direction of current flow through the head.
During a data read or a servo read operation, the suspension arm 16 is moved while the disc 12 is rotated to position the read/write head 14 above a magnetized region to be read. A DC (direct current) bias voltage of 0 volts to about 0.3V is supplied to the read/write head 14. The magnetized disc region changes a resistance of the magnetoresistive element in the read/write head 14, generating an output signal comprising a relatively small AC (alternating current) voltage imposed on the DC bias voltage.
The output signal is supplied to a read circuit 40A of a preamplifier 40. From the read circuit 40A, servo data is supplied to a servo read circuit 42A of a recording channel 42; read data bits are supplied to a data read circuit 42B. The servo read operations are interlaced with either a data read or a data write operation, as the servo feedback information is required during both operations to maintain proper position of the read/write head 14. Due to the low signal levels and high-frequency components in the read output signal, the preamplifier 40 is conventionally mounted proximate the read/write head 14, commonly on a circuit board constructed from flexible material.
As is conventional in the art, the preamplifier 40 further comprises a serial port configuration control register 40C that communicates with a controller 54 over a conductor 41 for providing control signals to the configuration control register 40C for establishing operating parameters of the preamplifier 40.
A servo logic circuit 50 receives processed and demodulated servo data from the servo read circuit 42A and translates this information into a format acceptable to a servo DSP (digital signal processing) processor 52 that executes servo control algorithms to control head position and movement according to head location commands received from the controller 54. Control commands supplied by the servo DSP processor 52 are delivered to a voice coil motor power amplifier 56 that in turn controls the voice coil motor 18 to drive the read/write head 14 in a closed feedback loop to maintain the desired head position on the disc 12. A spindle motor power amplifier 57 receives command signals from the servo DSP processor 52 to maintain the spindle speed at typically about 10,000 RPM.
During data read operations, the data read circuit 42B of the recording channel 42 delivers read data to the controller 54 over a buss 62. The controller 54 performs error detection and correction on the read data prior to supplying the data to a user interface, such as an interface to a computer or data processing device (e.g., SATA, SCSI, SAS, PCMCIA interfaces).
To write data to the disc 12, the controller 54 receives data to be written from the user interface for formatting and adding error detection/correction information. The processed data are supplied over a buss 64 to a data write circuit 42C of the recording channel 42. A write gate signal is also supplied by the controller 54 to the data write circuit 42C; from the data write circuit 42C the write gate signal is supplied to the write circuit 40B of the preamplifier 40. The data write circuit 42C also provides a write data signal, that represents the data bits to be written to the disc 12, to the data write circuit 40B. When the write gate signal is asserted, the preamplifier 40 is activated for write mode operation, during which the write circuit 40B causes current supplied to the write element of the read/write head 14 to alternate (i.e., change direction) under influence of the write data signal (representing the data bits to be written to the disc 12) between a positive state (to write a ‘1’, for example) and a negative state (to write a ‘0’, for example). The write current magnetizes the disc 12 to store the data bits. As is known by those skilled in the art, the designation of a positive state as a data ‘1’ and a negative state as a data ‘0’ is arbitrary and can be reversed. As will be described below, deassertion of the write gate signal initiates the demagnetize function according to the teachings of the present invention.
The recording channel 42 further comprises a servo-write circuit 42D that in response to signals received from the servo DSP processor 52, generates servo information for writing onto the disc 12 via the preamplifier write circuit 40B. The servo write circuit 42D is typically active only during manufacture of the disc drive to write servo information 34 on the disc 12.
To increase storage capacity, a disc drive may comprise a plurality of stacked parallel discs 12. A read/write head is associated with each disc to write data to and read user data and servo data from a top and bottom surface of each disc.
Ideally, upon conclusion of a write operation, the inductive write element of the read/write head 14 should not influence the head's MR read element during a subsequent read operation. In practice, however, if the write current in the write element ceases abruptly at the end of a write operation, the inductive writer tends to retain remnant magnetization within its ferromagnetic core, thus creating a residual magnetic field. Due to the proximate location of the MR read element and the inductive write element within the read/write head 14, the remnant magnetization can undesirably bias the read element, possibly distorting the read signal and causing errors in read bits. Demagnetizing (degaussing) the write element after a write operation reduces the remnant magnetization.
A further motivation for demagnetizing the write element is the need to avoid data erasure from the disk 12 by stray magnetic fields. In modern high-density recording, the small bit-cell sizes in the disc 12 are susceptible to thermal agitation. Over many revolutions of the disc, the presence of the residual field within the write element can hasten collapse of the bit-cell magnetization, causing data loss.
To reduce the remnant magnetization, it is desired to demagnetize the inductive writer of the read/write head 14 after a data write operation. This process, also referred to as degaussing, is accomplished by repetitively alternating the head current polarity, causing the inductive write element to switch between a north and a south magnetic pole, while decaying the head current to zero. The switching is accomplished by applying a series of bursts or transitions having a duration T (where T=1/(data frequency)) to the inductive write element. Switching the current direction in a controlled manner over a predefined number of magnetic pole transitions and decaying the head current from its full value (i.e., the current value during a write operation) to zero during the transitions causes the write element to execute successively smaller loops of its MH curve (i.e., the hysteresis curve relating the magnetic field (H) to the magnetization (M)), causing the remnant magnetization to decay to nearly zero. In its effect on the core domain structure of the writer head, the degauss process is analogous to an annealing operation.
Possible implementations of the demagnetizing operation include an analog approach using programmable analog time constants (i.e., time constants derived from resistor/capacitor (RC) components or current-charged capacitors) to provide the head current ramp down. Such an approach requires a synchronizing element to synchronize the current ramp down interval with the bursts or transitions, and is therefore sensitive to on-chip component values that determine the analog time constants. Known degaussing implementations are also limited in their ability to provide an arbitrary shape to the decay profile of the write current during the ramp down interval.